test: Testing

[lantoli]

Description

Adds a two-step acceptance test for the reported bug: creating an INFINITE cluster without shard_size_limit_gb and then adding it in a subsequent update produces "No changes. Your infrastructure matches the configuration." — the update is silently skipped when it is the only change.

The new TestAccClusterAdvancedCluster_infiniteShardSizeLimitAdd runs create-without → update-with shard_size_limit_gb = 1024 → import, so PR checks reproduce the issue against real Atlas before a fix lands.

Link to any related issue(s): CLOUDP-447675

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fix and verified my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

Test-only change; no changelog entry needed.